1. Field of the Invention
The present invention relates generally to a semiconductor chip package, and in particular to a circuit board having earth terminals for detecting the disconnection of bonding wires during a wire bonding process and a wire bonding apparatus using such a circuit board.
2. Description of the Related Arts
Wire bonding is the most widely used technology for chip interconnection in the plastic chip assembly process. A semiconductor chip and a board are connected to each other with wires using thermo-compression bonding, ultrasonic bonding or thermo-sonic bonding processes. The thermo-sonic bonding and thermo-compression bonding processes comprise a ball-wedge bonding step using gold wires. The processes employ a capillary that is driven upon an application of an ultrasonic-wave during every wire bonding process to improve an adhesion force between the gold wires and the aluminum bonding pads.
FIG. 1 is a schematic view showing a conventional wire bonding process between a lead of a lead frame and a semiconductor chip. Referring to FIG. 1, bonding pads (23) of a board (21) are connected to bonding pads (34) of a semiconductor chip (33) by wires in a BGA (Ball Grid Array) package (20).
The BGA package (20) comprises the circuit board (21) and the semiconductor chip (33). The circuit board (21) is composed of BT resin(Bismaleimide Triazine resin) and has a conductive pattern layer (22). After the wire bonding process, chip (33) is encapsulated with a thermosetting molding resin to create a package body (not shown). A chip-mounting part (31) for mounting the semiconductor chip (33) by adhesives (32) thereon, and bonding pads (23) for wedge bonding are formed on the upper surface of the circuit board (21). Solder ball pads (24) for mounting a plurality of solder balls (28) are formed and attached to a bottom surface of the circuit board (21) opposite to the surface where the bonding pads (23) are formed. The whole bottom surface of the circuit board (21), except the solder ball pads (24), is covered with a solder resist (27) in order to increase adhesion between the solder balls (28) and the board (21). The bonding pads (23) are electrically connected to the solder ball pads (24) through signal via (25). Heat-emitting via (26) releases heat generated from semiconductor chip (33) to the outside of the package body.
After mounting the semiconductor chip (33) on the circuit board (21), dampers (30) hold and fix the board (21). The surface of the board (21) to be held by the damper is first covered with a protective layer (29). The positions of the bonding pads (34) of the chip (33) and the positions of the bonding pads (23) of the board (21) are recognized and aligned by the bonding head (10) using a CCD camera (not shown).
The bonding head (10) comprises a spool unit (18) wound by wire (15) for the bonding process, a capillary (12) and a torch electrode (16). When the switch (19) of the spool unit (18) is turned on, the wire (15) has constant tension and is connected to the capillary (12) through a transducer (14).
The wire bonding process comprises a first ball bonding step for the aluminum bonding pad (34) of the semiconductor chip (33), and a second wedge bonding step for the bonding pad (23) of the board (21). A ball is formed at the end of the wire (15) when a high voltage, for example, over 5,000 Volts, is instantaneously applied to the end of the wire (15) from the capillary (12) by torch electrode (16). As shown in FIG. 2, the first ball bonding step is performed on the aluminum bonding pad (34) by pressing and heating the wire ball (35) on the bonding pad (34), thereby bonding the wire ball (35) on the bonding pad (34) of the semiconductor chip (33). Adhesion between the two metals may be increased by applying ultrasonic power at the time of the bonding process.
After the first ball bonding step, the wire (38) is guided to the bonding pad (23) of the board (21) by the capillary (12) and then the second wedge bonding step is performed on the bonding pad (23) as shown in FIG. 2. After the second wedge bonding step, the rest of the wire is cut, thereby completing one wire bonding cycle. This wire bonding process is repeated for every bonding pad to be connected.
The wire bonding process as described above is essential to ensure that power is input to the semiconductor integrated circuit and information is output from the semiconductor integrated circuit to an external terminal. However, as semiconductor devices become denser and more integrated, the number of input/output terminals to be bonded increases, and the need for a more advanced bonding technique is evident. However, a good wire connection, performed by the bonding process, can be still only be detected by visual inspection and reliability tests. If there are errors in the wire bonding process or impurities on the aluminum bonding pads (34), the wire may be disconnected. This problem is compounded in the case of highly-integrated semiconductor devices, which may have numerous wire disconnections, and where it is impossible to detect these disconnections during the wire bonding process. As a result, substandard articles are produced.